Until now, when a man skilled in the art intends to obtain a particular polypeptide or a cDNA encoding it, he generally utilizes methods by confirming an aimed biological activity in a tissue or in a cell medium, isolating and purifying the polypeptide and then cloning a gene or methods by “expression-cloning” with the guidance of the said biological activity. However, physiologically active polypeptides in living body have often many kinds of activities. Therefore, it happens increasingly that after cloning a gene, the isolated gene is found to be identical to that encoding a polypeptide already known. In addition, some factors could be generated in only a very slight amount and/or under specific conditions and it makes difficult to isolate and to purify the factor and to confirm its biological activity.
Recent rapid developments in techniques for constructing cDNAs and sequencing techniques have made it possible to quickly sequence a large amount of cDNAs. By utilizing these techniques, a process, which comprises constructing cDNAs library using various cells or tissues, cloning the cDNA at random, identifying the nucleotide sequences thereof, expressing novel polypeptides encoded by them, is now in progress. Although this process is advantageous in that a gene can be cloned and information regarding its nucleotide sequence can be obtained without any biochemical or genetic analysis, the target gene can be discovered thereby only accidentally in many cases.
The present inventors have studied cloning method to isolate genes encoding proliferation and/or differentiation factors functioning in hematopoietic systems and immune systems. Focusing their attention on the fact that most of the secretory proteins such as proliferation and/or differentiation factors (for example various cytokines) and membrane proteins such as receptors thereof (hereafter these proteins will be referred to generally as secretory proteins and the like) have sequences called signal peptides in the N-termini, the inventors have conducted extensive studies on a process for efficiently and selectively cloning a gene encoding for a signal peptide. Finally, we have successfully developed a screening method for the signal peptides (signal sequence trap (SST)) by using mammalian cells (See Japanese Patent Application No. Hei 6-13951). We also developed yeast SST method on the same concept. By the method based on the same conception using yeast, (yeast SST method), genes including sequence encoding signal peptide can be identified more easily and efficiently (See U.S. Pat. No. 5,536,637).